Photodetectors are an integral part of optical circuits and components (for example emitters, modulators, repeaters, waveguides or fibers, reflectors, resonators, detectors, IR Focal plane arrays, etc.) and are used for the sensing of electromagnetic radiation. There are several approaches to these devices. Photoconducting materials, typically semiconductors, have electrical properties that vary when exposed to electromagnetic radiation (i.e. light). One type of photoconductivity arises from the generation of mobile carriers (electrons or holes) during absorption of photons. For semiconducting materials, the absorption of a specific wavelength of light, hence photon energy, is directly proportional to the band gap of the material (Eg=hn=hc/l, where Eg is the materials band gap, h is Plank's constant (4.136×10−15 eVs), c is the speed of light in a vacuum (2.998×1010 cm/s) and l is the wavelength of the radiation). If the band gap energy is measured in eV (electron Volts) and the wavelength in micrometers, the above equation reduces to Eg=1.24/l. A photodiode (i.e. p-n diode, p-i-n photodiode, avalanche photodiode, etc.) is the most commonly employed type of photoconductor.
Light detection is ideally suited for direct band gap semiconductors such as Ge, GaAs, etc.; however, indirect band gap semiconductors (where an additional phonon energy is required to excite an electron from the valence band to the conduction band), such as Silicon, are also used as photodetectors. Probably the most widely known type of photodetctor is the solar cell, which uses a simple p-n diode or Schottky barrier to detect impinging photons. Besides silicon, most photodetectors do not integrate with current microelectronics technology, usually detect only a specific wavelength (i.e. 1.1 mm for Si, 0.87 mm for GaAs, 0.414 mm for a-SiC and 1.89 mm for Ge), and require multiple detectors to detect a broad band of wavelengths (hence photon energy).
There are other types of photodetectors that do not rely on the generation of current through the excitation of electrons (or holes). One such type of detector is the bolometer. Bolometers operate by absorbing radiation, which in turn raises the temperature of the material and hence alters the resistance of the material. Bolometers can be constructed from either metallic, metallic-oxides or semiconducting materials such as vanadium oxide, amorphous silicon. Since bolometers detect a broad range of radiation above a few microns, bolometers are typically thermally stabilized to reduce the possibility of detection of blackbody radiation that is emitted from the detector material, which leads to a high background noise. IR microbolometer detectors and arrays don't require cooling to cryogenic temperatures unlike the other detector technologies discussed. Another type of non-photo-generated detector is the pyroelectric detector. Pyroelectric detectors operate by sensing induced surface charges that are related to changes in the internal dipole moment generated from temperature shifts in the material.
It is possible for IR and visible light to be detected from individual single-walled nanotubes (SWNTs). Carbon nanotubes possess discrete absorption peaks that correspond to specific photon energies. For useful background material, refer to U.S. Pat. No. 6,400,088. As described, the absorption peaks of the carbon nanotubes correlate directly to the diameter of the carbon nanotube.
Typical band-gaps for carbon nanotubes (CNTs) range from 0.6-1.2 eV, depending on the diameter of the CNT, where the band gap is proportional to the inverse diameter of the nanotube. These energies correlate to the nanotubes ability to detect radiation in the near IR range. Since nanotubes can also generate heat and phonons by several processes (injection of electrons, impinging with radiation, etc.), a CNT fabric is also ideally suited as an IR detector.
The current state of the art micro bolometer utilizes vanadium oxide as the element which changes impedance for incoming IR radiation. Typically 2% per degree Centigrade is the highest thermal coefficient of resistance achievable. This performance is restricted by 1/f noise and the basic physical properties of the vanadium oxide (VOx) film. The VOx based micro bolometer is fabricated on top of the CMOS readout circuit, which provides a cost benefit.
There is a need for light detectors that use nanotubes and methods of making the same which addresses the issues described above.